1. Field of the Invention
The present invention relates to an image forming apparatus including a charging device using a magnetic brush charging method.
2. Description of the Related Art
There are many image forming apparatuses referred to as an electrophotographic type or an electrostatic recording type, which are conventionally proposed. FIG. 1 illustrates a representative configuration of a conventional image forming apparatus. An operation of the image forming apparatus is simply described below with reference to FIG. 1.
The image forming apparatus illustrated in FIG. 1 includes a corona charger 3 for a photosensitive drum 1 that has a predetermined potential at the surface thereof when a copy start signal is input. An integral unit 9, including a document illumination lamp, a short focal lens array, and a charge-coupled device (CCD) sensor, irradiates a document G on a document placing glass plate 10 with light to perform scanning. The light reflected from the document surface (i.e., part of the irradiated scanning light) reaches the CCD sensor via the short focal lens array that forms an image of the document on the CCD sensor.
The CCD sensor includes a light-receiving portion, a transfer portion, and an output portion. The light-receiving portion converts light incident on the CCD sensor into an electric charge signal. The transfer portion successively transfers the electric charge signal from the light-receiving portion to the output portion in synchronism with a clock pulse. The output portion converts each electric charge signal into a voltage signal, and outputs an amplified and impedance-reduced analog signal to an external device. The analog signal thus obtained is subjected to conventional image processing to produce a digital signal and is transferred to a printer unit. The printer unit includes a light emitting diode (LED) exposure unit 2 configured to selectively emit light in response to an image signal and form an electrostatic latent image corresponding to a document image on a surface of a photosensitive drum 1.
A development unit 4 accommodating toner particles is configured to develop the electrostatic latent image and obtain a toner image on the photosensitive drum 1. The development unit 4 coats a developer on a development sleeve 41 accommodating magnet rollers in its inner space. The development unit 4 includes a power source (not illustrated) capable of applying a development bias to develop toners on the photosensitive drum 1. A transfer device 7 electrostatically transfers the toner image formed on the photosensitive drum 1 to a transfer member. Then, the transfer member is electrostatically separated and conveyed to a fixing unit 6 configured to output a thermally fixed image.
After the toner image transfer operation is completed, a cleaner 5 removes the toners not transferred and remaining on the surface of the photosensitive drum 1. The cleaner 5 can also remove any contaminant substances from the surface of the photosensitive drum 1. If necessary, the surface of the photosensitive drum 1 is exposed to light from a pre-exposure lamp 8 serving as a pre-exposure unit configured to remove an image exposure light memory so that the photosensitive drum 1 can be repetitively used for image formation.
The corona charging method is generally used for the above-described image forming process, i.e., for an electrophotographic image forming apparatus. However, enthusiastic study and development are recently performed for the contact charging method that can reduce an amount of ozone products (i.e., discharge products) and can operate at a lower power level. And, there are some contact charging systems already used.
The contact charging method is a charging method for charging a photosensitive member by causing a charging member to contact the photosensitive member and applying a voltage to the charging member. A magnetic brush charging device using a magnetic brush as a contact charging member is a representative charging device using the above-described method. The magnetic brush charging device is advantageous in that charging contact is stabile.
The magnetic brush charging device can magnetically hold conductive magnetic particles directly on a surface of a magnet, or on a surface of a sleeve accommodating a magnet, and can cause the magnetic particles to contact a surface of a photosensitive member and apply a voltage to charge the photosensitive member. When the magnetic brush charging device is used to charge a photosensitive member having a surface layer including diffused conductive fine particles on a conventional organic photosensitive member, or an amorphous silicon photosensitive member, the charging operation can be performed using a charging potential substantially equivalent to a DC component of a bias applied to the magnetic brush. This charging method is hereinafter referred to as a magnetic brush injection charging method.
The magnetic brush injection charging method does not use any discharge phenomenon similar to that used in the corona charging method, in a charging operation for a photosensitive member. Accordingly, an ozoneless and low-power consuming charging operation can be realized. Flow of an image derived from a discharge product does not appear even in a high-humid environment.
Compared to the organic photosensitive member, the amorphous silicon photosensitive member has a higher hardness and a long life. The running cost of a product can be reduced. As understood from the above description, a combination of the magnetic brush injection charging method and the amorphous silicon photosensitive member can realize an image forming system excellent in both durability and stability.
However, according to the magnetic brush injection charging method, conductive magnetic particles stored in a charging container are subjected to abrasion on their surfaces, electric breakdown, and inclusion of foreign particles entering the magnetic brush (e.g., a developer entering via a cleaning blade). Therefore, electric properties and powder properties of the magnetic particles change during a long-term use.
More specifically, the property of a coat on a sleeve gradually deteriorates due to an increase in the resistance and a change in the flowability of the magnetic particles. The above-described deteriorations occurring in the magnetic particles are inevitable. Therefore, a user is required to perform a replacement work for replacing the magnetic particles at appropriate timing during a long-term use of an image forming apparatus.
Similar deteriorations in conductive magnetic particles are recognized in a two-component development method. The two-component development method is a widely used conventional development method applicable to an electrophotographic image forming apparatus, particularly to an image forming apparatus performing chromatic image forming processing, according to which a mixture of non-magnetic toners and magnetic particles (development carrier) is used as a developer. Similar deterioration is recognized in the developer.
When the agent (e.g., charging magnetic particles or developer) is deteriorated, an agent replacement operation is performed. In the agent replacement operation, it is not so difficult to always equalize a discharge amount of a discharge unit discharging the deteriorated agent with a replenishment amount of a replenishment unit replenishing a new agent. However, the discharge amount and the replenishment amount may not be identical if the discharge amount varies or if replenishment accuracy deteriorates.
In a two-component development device performing discharge and replenishment of the developer, if a discharge amount of the developer is larger than a replenishment amount of a new developer, the developer surface in a development unit gradually lowers. If the developer decreases greatly, no developer can be supplied to a development sleeve.
On the contrary, if the discharge amount of the developer is less than the replenishment amount of a new developer, an excessive amount of developer may be supplied to the development sleeve or part of the developer may leak out of the development unit.
There are some conventional methods capable of solving the above-described problems in the two-component development method. For example, as discussed in Japanese Patent publication No. 2-21591, a development apparatus usable for an electrophotographic copying machine includes a developer discharge unit and a developer replenishment unit, including an agitating unit agitating carriers and toners and a development roller supplying the developer agitated by the agitating unit to a photosensitive member. A carrier replenishment apparatus and a toner replenishment apparatus are provided, separately or integrally, above the agitating unit. A developer overflow portion is provided on a sidewall of the development apparatus. Therefore, while the replenishment apparatus replenishes a new developer by degrees, excessive developer can be discharged from the developer overflow portion. The developer in the development apparatus can maintain constant properties. As a result, printed products can possess constant image quality.
More specifically, the apparatus replenishes the developer while regulating the particle surface level in an agitating region to gradually replace the deteriorated developer with new developer, thereby preventing the agent from deteriorating and stabilizing the properties. The apparatus does not require any work for replacing the developer and can realize an improved maintenance operation.
As discussed in Japanese Patent Application Laid-Open No. 2003-330270, there is another method for discharging carriers using a fogging removal potential so as to develop the carriers transferred from a development sleeve to a photosensitive member in a non-image region on the photosensitive member and supplying the developer to the development sleeve in a region other than the region where the carriers are discharged.
The development particle surface level in the agitating region is generally set to a height sufficient to supply the developer in the entire range of the development sleeve in the longitudinal direction. Therefore, the developer is supplied to the carrier discharge region on the development sleeve. The carriers adhere to the photosensitive member at a development nip portion, and a carrier discharge operation is performed. However, if the development particle surface level in the agitating region is lower than a predetermined height, the developer is not supplied to the development sleeve only in the carrier discharge region. Therefore, no carriers can be discharged at the development nip portion. Therefore, the lower movement of the development particle surface is stopped. The predetermined height of the development particle surface is a height where the developer cannot be supplied to the development sleeve in any region other than the carrier discharge region.
Namely, the development particle surface can be regulated to a constant level in an area where the magnetic force can act from the development sleeve to a developer agitating region. As described above, the development apparatus using the two-component agent including toners and carriers can regulate the height of the particle surface level using the gravity in the developer agitating region where no magnetic force acts from development sleeve, because the development apparatus includes the agitating unit agitating the toners and the carriers. Furthermore, the development apparatus can hold a constant amount of developer. Accordingly, the development apparatus does not require any work for replacing the developer and can realize an improved maintenance operation. The development apparatus can stably adjust the amount of the developer with a simple configuration.
However, compared to the above-described two-component development device configured to agitate toners and carriers, the magnetic brush charging device is not required to agitate the agent. The magnetic force of the charging sleeve acts on almost all of the magnetic particles. The above-described method for regulating the particle surface by the gravity cannot be used. If a region where no magnetic force acts is necessary, it is required to provide an additional area. Furthermore, an agitating member is required to uniformly set the particle surface in the longitudinal direction.
If the magnetic brush charging device employs a conventional carrier replacement unit widely used for two-component development apparatuses, it is required to provide an agitating mechanism and secure a sufficient space. The cost and the size of the apparatus increase. Moreover, even in a case where the agitating mechanism is provided, accurately controlling the height of the particle surface level is desired.